Defective intestinal epithelial tight junction (TJ) barrier is a key pathogenic factor of inflammatory bowel disease (IBD) and other inflammatory conditions of the gut. The defective TJ barrier allows increased intestinal permeation of bacterial antigens that induce inflammatory response. However, there are no currently available therapeutic agents that target the intestinal TJ barrier. Moreover, the mechanisms that lead to an enhancement of intestinal TJ barrier remain poorly understood. The major goals of this proposal are to introduce and develop a new therapeutic agent that targets the intestinal TJ barrier, which can be rapidly advanced for clinical usage; and to identify novel intracellular mechanisms that regulate the tightening of the intestinal TJ barrier. In our preliminary studies, we screened over 20 probiotic bacterial strains (that are widely used commercially) to identify a single strain, Lactobacillus acidophilus (LA), which has a remarkable intestinal TJ barrier augmenting properties and therapeutic efficacy in animal models of IBD. The over-arching goals of this application are to investigate the intestinal TJ barrier enhancing effects of LA and to determine the therapeutic efficacy of LA in animal models of IBD. Based on our preliminary studies, we advance a novel hypothesis that LA enhancement of intestinal TJ barrier is regulated by a nucleotide-binding oligomerization domain - containing protein 1 (Nod1) signal transduction pathway activation of occludin gene. Occludin is a trans-membrane protein which plays a central role in intestinal TJ barrier regulation. In this grant application, we challenge to well-established scientific paradigms 1) that Nod1 is a cytoplasmic pattern recognition receptor and 2) that primary cellular target of Nod1 is the activation of NF-?B. Based on our preliminary data showing that LA induces a rapid cytoplasmic-to-apical membrane translocation of Nod1 in intestinal epithelial cells in-vitro and in-vivo, we hypothesize that LA- induced augmentation of intestinal TJ barrier is regulated by apical membrane translocation of Nod1. We also hypothesize that LA activation of Nod1 signal transduction pathway leads to a suppression (not activation) of NF-?B, a key pro-inflammatory mediator that induces the TJ opening, and an activation of p38 kinase pathway, which signals occludin gene activation and occludin-dependent enhancement in intestinal TJ barrier function. The proposed specific aims are to: 1) delineate the role of Nod1 in LA-induced augmentation of intestinal epithelial TJ barrier; 2) delineate the mechanism of LA-induced regulation of occludin gene and intestinal TJ barrier; and 3) delineate the therapeutic efficacy of LA in animal models of IBD. The successful completion of the proposed studies will provide the critical pre-clinical data necessary to support our future clinical trials that will examine the therapeutic efficacy of LA in intestinal barrier tightening in IBD patients and also in the prevention and treatment of IBD.